This invention relates to oil and gas well logging and in particular provides a system for collecting real time and lag data concerning the drilling operations of an oil and gas well which are then processed and correlated to match real time data at a particular elevation with lag data and then displayed on a screen or printed out for immediate utilization in the drilling operation. The data are processed and correlated at preselected increments of well elevation and provide a geological-hydrocarbon log therefor.
The usual procedure in oil well drilling operations is to pump a drilling fluid or mud down through the drill stem and into the region around the drill bit during drilling. The mud then flows back up to the surface through the well bore outside the drill stem. The drilling mud is typically made up of clays and chemical additives in an oil or water base and performs several important functions. The drilling mud acts as a coolant and lubricates the drill bit during operation and it collects the drill bit cuttings and carries them back to the surface of the well. The drilling mud also serves to maintain a hydrostatic pressure to prevent pressurized gases from the earth from blowing out through the well during drilling. Further, the drilling mud will pick up and entrain gases present in the bottom of the well and deliver them to the surface along with the drill cuttings.
In oil and gas drilling operations a log of the operation is maintained that will permit the nature of the earth formation through which the drill bit is penetrating to be analyzed. The log is important because it enables the drilling operator to ascertain the presence of oil or gas in the formation being drilled and also the location or elevation of such oil or gas in the well. As part of this logging operation, samples of the drill cuttings from predetermined depths of the well are collected and analyzed. Generally, these samples will be collected to represent a desired interval of drilling, such as every ten feet of well drilled or every thirty feet drilled.
The logging of a well, frequently called mud logging because the information recorded and drill cuttings collected and analyzed are obtained from the drilling mud when it reaches the surface of the well, is generally done on a manual basis by a person called a "mud logger". This person, usually with a background in geology, collects and analyzes the well drill cuttings obtained from the drilling mud, monitors the gases released from the drilling mud, and enters the information collected in a well log along with information concerning the depth of the well where the cuttings and gas originated. Thus, the mud logger keeps track of the depth of the well generally through the use of a depth measuring device on the drilling rig called a geolograph and, having an approximate idea of the lag time, i.e., the time it takes mud at the bottom of the well to reach the top of the well, gained through occasional rough measurements of the lag time, estimates the depth from which a mud sample reaching the surface originated. All of the measurements and the measuring equipment require constant supervision so a logging operation generally involves two mud loggers each working alternate twelve hour shifts.
Lag time, the time required for drill bit cuttings and drill bit liberated formation gas entrained in the drilling mud to reach the surface after being drilled is generally determined every day or more, or every several hundred feet of drilling or more if desired by determining the amount of time required for a marker device, i.e., a small packet of carbide, multicolored rope, etc., to appear in the drilling fluid return line after being dropped into the drill pipe. This customary method is inaccurate in determining lag time because of the relative infrequency of the measurement and because a measurement based merely on time cannot accurately allow for changes in drilling fluid pump speed or for changing from one pump to another and the resulting change in pump output.
The measurement of lag is an important aspect in determining when samples of drill cuttings are to be collected for further examination. Samples, which comprise formation cuttings strained from the circulating drilling fluid as it reaches the surface of the well, are generally collected for microscopic examination at various increments such as every ten feet or so of well penetration. These samples are then representative of the drilling cuttings produced through that increment. Sample collection times are generally determined by adding the lag time to the time when samples are drilled. Therefore, if a sample of drill stem cuttings from a well depth of 3100 feet has been collected and it is desired to collect samples at ten foot intervals, the next sample should be collected when mud from a depth of 3110 feet reaches the surface. The mud logger would determine from drilling operations when the well was at a depth of 3110 feet, for example at 7:00 a.m., and then would add the lag time to determine when the drill bit cuttings should reach the surface and a sample be collected. If the lag time had been determined by measurement to be one-half hour, the mud logger would add one-half hour to the 7:00 a.m. time and know that the next sample of drill cuttings should be collected at 7:30 a.m. and, if collected at that time, the drill cuttings should be those cut between the 3100 and the 3110 depth level of the well. It follows if the lag time is not accurately determined, the samples of drill bit cuttings collected are not representative of the desired depth. Further, merely adding the approximately determined lag time to the time the well reaches the desired depth does not allow for compensation necessary if, during that time, the drilling fluid pump speed changes, the pump used to pump the fluid changes, or the drilling rig temporarily stops drilling.
A computerized system for keeping track of drilling mud sampling times to obtain drill cuttings is described in Larry J. Gunther. U.S. Pat. No. 4,860,836. This patent describes a computerized technique for determining sample collection times which continually monitors parameters necessary to determine the depth at which drilling mud reaching the surface of a well was injected into the well and thus aids in the sampling of the drilling mud when it reaches the earth's surface. Likewise, the system monitors the performance of well personnel in collecting samples by monitoring the time it takes after the indication that the sample should be collected.
During oil and gas well drilling operations it is of interest to determine the formation characteristics such as permeability as the drilling operation progresses. In Daniel E. Boone U.S. Pat. No. 4,961,343 describes a system for determining permeability. Boone monitors the volume of hydrocarbon gas pressure in the return drilling fluid to obtain the percentage of gas saturation in the formation. The rate of penetration of the drill bit is monitored. A pore saturation function is derived wherein anomalously low values thereof relative to expected values for productive zones indicate flushing ahead of the drill bit of hydrocarbons out of the pore volume of cuttings in the return fluid. A mud filtrate flow rate is then determined from the drilling rate, gas saturation, and formation porosity. Pressure differential in the vicinity of the drill bit is also determined, comprised of the difference between the drilling fluid column pressure and pore pressure. Formation permeability is then directly determined in real time during the drilling operation from a functional relationship between the filtrate flow rate, pressure differential and viscosity of the drilling fluid. Boone of course is interested in a more narrow view of the geological and hydrocarbon characteristics of the well being drilled.
It has become increasingly desirable for real time and historical logging data to be generated in a form such that it can be conveniently correlated to provide information not easily obtained by mere comparative visual inspections. Boone and Gunther provide only partial solutions to fulfilling these needs of the industry.